The present invention relates generally to electrical circuits and, more particularly, to power supply control circuits, to control for example the switching of field effect transistors for power-supply sequencing applications.
Various techniques may be employed to control when a printed circuit board or a portion of a system is supplied with power. For example, one technique employs power MOSFET (metal oxide semiconductor field effect transistor) devices to effectively act as electronic switches that control when power is applied to the printed circuit board or the portion of the system. Typically, N-channel MOSFETs (also referred to as NFETs or power NFETs) are utilized because of their lower on-resistance and lower cost.
When positive power supplies are switched with these power NFETs, it is necessary to supply a gate voltage to the power NFETs that exceeds the supply voltage they switch by several volts. However, FETs (e.g., NFETs or other types of MOSFETs) are sensitive to over-voltage at their control gates and some form of clamping or voltage monitoring must be provided to avoid possible destruction of the FETs.
Furthermore, the rate at which the NFETs are to be switched on is critical. If switched on too fast, load currents rush into the system or printed circuit board at an excessive rate, which may force the master power supply to go into limit mode or cause various reliability or malfunction problems. If switched on too slowly, the NFET devices may remain in a high-resistance mode for too long and heat up to a point where they can self-destruct.
There are numerous types of power NFETs having different characteristics and requirements for voltage and current parameters. Conventional integrated circuits that control these power NFETs generally require the use of external clamping (i.e., voltage protection) devices to limit the gate voltage along with external shunt capacitors to slow down the turn-on time and avoid exceeding power NFET safe operating limits. These additional discrete and specialized components, that are external to the integrated circuit, occupy valuable printed circuit board space, add to the manufacturing cost, and allow limited control, flexibility, or field programmability. As a result, there is a need to provide improved power supply control circuits.
Power supply control circuits and methods are disclosed herein. For example, in accordance with an embodiment of the present invention, a programmable current source with voltage compliance regulation through programmable feedback is provided to control one or more external power NFETs for power-supply sequencing applications. The programmable current source is combined with a programmable regulated boost supply (e.g., a high-voltage charge pump) to provide a fully integrated solution. Furthermore, with the turn-on ramp rate and the maximum output voltage programmable, a user can select the most desirable settings and operate the power NFET safely within its rated limits. Consequently, external clamping or shunting devices are not required.
Alternatively, in accordance with one or more embodiments of the present invention, either a current source or a regulated boost supply may be programmable. Furthermore, a current sink and/or an open drain circuit may be provided, with the open drain circuit providing an open drain logic output configuration if an output from the current source is not provided.
More specifically, in accordance with one embodiment of the present invention, a circuit includes a regulated boost supply, adapted to provide a programmable supply voltage, and a current source, coupled to the regulated boost supply, adapted to receive the programmable supply voltage and provide a programmable current.
In accordance with another embodiment of the present invention, a method of switching a transistor for power sequencing applications includes providing a programmable voltage that determines a desired voltage applied to a gate terminal of the transistor and providing a programmable current that receives the programmable voltage and determines a desired voltage ramp rate at the gate terminal of the transistor.
In accordance with another embodiment of the present invention, a circuit includes a regulated boost supply adapted to provide a supply voltage, and a current source, coupled to the regulated boost supply, adapted to receive the supply voltage and provide a programmable current.
In accordance with another embodiment of the present invention, a circuit includes a regulated boost supply adapted to provide a programmable supply voltage, and a current source, coupled to the regulated boost supply, adapted to receive the programmable supply voltage and provide a current.
In accordance with another embodiment of the present invention, a circuit includes a regulated boost supply adapted to provide a supply voltage, the boost supply including a feedback loop responsive to a reference voltage for maintaining a desired setting for the supply voltage, and a current source, coupled to the regulated boost supply, adapted to receive the supply voltage and provide a current.
In accordance with another embodiment of the present invention, a circuit includes a regulated boost supply adapted to provide a supply voltage, and a current source, coupled to the regulated boost supply, adapted to receive the programmable supply voltage and a reference current and provide an output current that is a multiple of the reference current.
The scope of the invention is defined by the claims, which are incorporated into this section by reference. A more complete understanding of embodiments of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly.